The complement of this first principle is the proposition that no thing is annihilated, but all existences are resolved into their ultimate elements. As the first is a necessary inference from the existence of universal order, the second is proved by the perpetuity of creation and the observed transformation of things into one another. The original substances out of which all things are produced, and into which they are ultimately resolved, are found to be certain primordial particles of matter or atoms, which are called by various names--'materies,' 'genitalia corpora,' 'semina rerum,' 'corpora prima.' Some of these names, it may be observed, are expressive not only of their primordial character, but also of a germinative or productive power. The objection that these atoms are invisible to our senses is met by showing that there are many invisible forces acting in Nature, the effects of which prove that they must be bodies,-- Corporibus caecis igitur natura gerit res. In addition to bodily substance there must also be vacuum or space; otherwise there could be no motion in the universe, and without motion nothing could come into being. The existence of matter is proved by our senses, of vacuum by the necessity of there being space for matter to move in, and also by the varying density of bodies. But besides body and vacuum there is no other absolute substance-- Ergo praeter inane et corpora tertia per se Nulla potest rerum in numero natura relinqui.[379] All material bodies are either elemental substances or compounded out of a union of these substances. The elemental substances are indestructible and indivisible. This is proved by the necessities of thought (i. 498, etc.) and of Nature. If there were no ultimate limit to the divisibility of these substances, if there were not something immutable underlying all phenomena, there could be no law or order in the world. The existence and ultimate constitution of the atoms is thus enunciated-- Sunt igitur solida primordia simplicitate Quae minimis stipata cohaerent partibus arte. Non ex illarum conventu conciliata, Sed magis aeterna pollentia simplicitate, Unde neque avelli quicquam neque deminui jam Concedit natura reservans semina rebus.[380] At this stage in the argument, from line 635 to 920 of Book I, the first principles of other philosophies, and particularly of the systems of Heraclitus, Empedocles, and Anaxagoras, are discussed at considerable length, and shown to be inconsistent with the actual appearance of things and with the principles already established.